Seatbelt restraint systems for restraining an occupant in a vehicle seat play an important role in reducing occupant injury in vehicle crash situations. Seatbelt restraint systems of the conventional so-called “3-point” variety commonly have a lap belt section extending across the seat occupant's pelvis and a shoulder belt section crossing the upper torso, which are fastened together or are formed by a continuous length of seatbelt webbing. The lap and shoulder belt sections are connected to the vehicle structure by anchorages. A belt retractor is typically provided to store belt webbing and may further act to manage belt tension loads in a crash situation. Seatbelt restraint systems which are manually deployed by the occupant (so-called “active” types) also typically include a buckle attached to the vehicle body structure by an anchorage. A latch plate attached to the belt webbing is received by the buckle to allow the belt system to be fastened for enabling restraint, and unfastened to allow entrance and egress from the vehicle. Seatbelt systems, when deployed, effectively restrain the occupant during a collision.
OEM vehicle manufacturers often provide seatbelt restraint systems with pretensioning devices, which tension the seatbelt either prior to impact of the vehicle (also known as a “pre-pretensioner”) or at an early stage of a sensed impact to enhance occupant restraint performance. The pretensioner takes out slack in the webbing and permits the belt restraint system to couple with the occupant early in the crash sequence. One class of pretensioning devices is a pyrotechnic linear pretensioner (PLP) which can be implemented as a pyrotechnic buckle pretensioner (PBP) which is attached to a seat belt buckle. PLPs can also be attached to a webbing guide loop or seatbelt anchorage. Since both types pull a seat belt system component linearly to apply tension in the belt webbing, both PLPs and PBPs can be collectively referred to as a PLP. Examples of designs of PLPs and PBPs are provided by U.S. Pat. No. 6,068,664, which is hereby incorporated by reference. When a collision occurs, a pyrotechnic charge of a PLP is fired, producing expanding gas which pressurizes a gas chamber within a tube, which forces a piston down the tube. The piston is connected with the belt system by a cable or strap. Stroking of the piston tightens or “pretensions” the belt against the occupant.
PLPs in accordance with present design approaches typically utilize a machined or die cast combustion chamber and a tubular piston cylinder formed of aluminum or steel. The parts are typically threaded or staked together. In addition, there are mounting features formed as part of the combustion chamber or stampings are fastened to the die cast combustion chamber. Although these devices operate satisfactorily, the requirement of die casting or machining of a solid metal blank results in a relatively heavy component and requires costly tooling and equipment for production. Designers of automotive components are constantly striving to reduce the mass of the components as part of the goal of providing higher fuel efficiency for motor vehicles. Moreover, cost is a primary consideration for automotive components.